study design and research population · BEGINNER LESSON
Cohort
A cohort is a group of people or subjects who share a common experience, characteristic, or selection criteria. In research, a cohort is followed over time so investigators can measure outcomes and compare what happens in exposed and unexposed groups.
What a cohort is
A cohort is a group of people or subjects who share something in common, such as a birth year, a condition, or an exposure. In research, the group is observed over time and data are collected so investigators can measure later outcomes, such as illness, death, or other events. [1,2,3]
[1] pmc.ncbi.nlm.nih.gov › articles › PMC4214579An Introduction to the Fundamentals of Cohort and Case ... [2] Cohorts as collections of bodies and communities of persons [3] Research Design: Cohort Studies - PMC - NIH
How cohort studies work
A cohort study follows two or more groups forward in time, often comparing people who are exposed to something with people who are not exposed. This design is longitudinal, meaning it includes follow-up, and it can be prospective, retrospective, or both depending on how the cohort was defined. [0,2]
[1] pmc.ncbi.nlm.nih.gov › articles › PMC4214579An Introduction to the Fundamentals of Cohort and Case ... [3] Research Design: Cohort Studies - PMC - NIH
What is measured
Cohort studies can directly calculate incidence, which means the number of new outcomes in a group over time. They are used to estimate associations between an exposure and an outcome, and they can study multiple outcomes from one exposure. [0,2]
[1] pmc.ncbi.nlm.nih.gov › articles › PMC4214579An Introduction to the Fundamentals of Cohort and Case ... [3] Research Design: Cohort Studies - PMC - NIH
Mechanism and strengths
The key idea is time order: the exposure happens before the outcome, so the study can show whether an outcome followed an exposure. Cohorts are especially useful for rare exposures and for studying how risk changes across time. [0,2]
[1] pmc.ncbi.nlm.nih.gov › articles › PMC4214579An Introduction to the Fundamentals of Cohort and Case ... [3] Research Design: Cohort Studies - PMC - NIH
Limitations
Because cohort studies are observational, people are not randomly assigned to exposure groups. That means confounding, which is when another factor distorts the relationship, and different kinds of bias can make an association look stronger or weaker than it really is. Long follow-up can also lead to dropout, missing data, and less accurate records. [0,2]
[1] pmc.ncbi.nlm.nih.gov › articles › PMC4214579An Introduction to the Fundamentals of Cohort and Case ... [3] Research Design: Cohort Studies - PMC - NIH
SEE THE IDEA
How a cohort study is organized
People with a common trait or experience are identified.
The same people are observed at baseline and later points.
People with and without the exposure are compared.
New events are counted and compared as associations.
Shared group -> followed over time -> split by exposure -> outcomes measured and compared
A cohort begins as a defined group and becomes a study by tracking what happens next.
TRY IT YOURSELF
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Sources and further reading
This general lesson was generated from public source material. It explains a concept; it does not interpret a private paper or provide medical advice.